Cavity resonator for MR systems

ABSTRACT

An magnetic resonance apparatus in embodiments of the invention may include one or more of the following features: (a) a coil having at least two sections, (b) the at least two sections having a resonant circuit, (c) the at least two sections being wirelessly coupled or decoupled, (d) the at least two sections being separable, (e) several openings allowing a subject to see and be accessed through the coil, (f) at least one cushioned head restraint, and (g) a subject input/output device providing visual data from in front and behind of the coil respectively; wherein the input/output device is selected from the group consisting of mirrors, prisms, video monitors, LCD devices, and optical motion trackers.

This application claims priority to U.S. Provisional Application Ser.No. 60/381,356 titled “Cavity Resonator for NMR Systems” filed on May17, 2002.

FIELD OF THE INVENTION

The present invention relates to a cavity resonator coil for use inmagnetic resonance systems.

BACKGROUND OF THE INVENTION

There are many previously known resonators for use in magnetic resonance(MR) systems for imaging and spectroscopy. For example, one previouslyknown device is commonly known as a birdcage resonator. These previouslyknown birdcage resonators typically comprise a plurality ofcircumferentially spaced inductive/capacitive elements connected byinductive/capacitive end ring elements, which are driven at resonance bya Larmor radio frequency useful for nuclear magnetic resonance (NMR)systems. The object to be analyzed, e.g. the brain, is positioned withinthe resonator during the operation of the MR system.

One disadvantage of these previously known resonators, however, is thatthey typically use lumped element circuits (with discrete inductors andcapacitors) to achieve resonance at the selected radio frequency.Because lumped element circuits lose more energy to radiation at highfrequencies where the circuit is greater than 1/10 wavelength, thelumped element resonator is less efficient for high field MR imagingapplications compared to lower field strengths. Because lumped elementcircuits are more radiative, they are electrically less efficient andhave a lower Q factor. Similarly because conventional lumped elementcircuits such as the birdcage are more inductive, they resonate at lowerfrequencies than do the less inductive transmission line (TEM) circuits.

These previously known resonators, which use lumped element circuits,suffer from several additional disadvantages. One such disadvantage isnon-uniform current distributions which result in decreased homogeneity,decreased fill factor, and increased electric field losses. Especiallyat higher frequencies and for larger (clinical) volumes, lumped elementresonators may achieve self-resonance below the desired frequency ofoperation as well as increased electromagnetic radiation losses.

A distributed circuit, cavity resonator for NMR systems disclosed byVaughan in U.S. Pat. No. 5,744,957 overcomes the above-mentioneddisadvantages of the previously known devices. Vaughan discloses acavity resonator having coaxial inner and outer cylindrical wallsseparated by a dielectric region. Spaced conductive end walls extendacross the inner and outer walls at each axial end of the coil so thatthe inner, outer, and end walls together form an approximate cavity. Theinner, outer, and end walls can, for example, be coated with a thin foilconductive material, such as copper, silver, or gold. In doing so, thecoil imitates a coaxial line segment made resonant at Larmor frequenciesuseful for MR, such as 64 MHz (1.5 T), 175 MHz (4.1 T) or 170 MHz (4 T).

The apparatus disclosed by Vaughan provides a cavity resonator coilovercoming the problems of conventional coils discussed above, providingfor a substantial improvement in signal to noise ratio (SNR). The coilwill also resonate and operate efficiently at higher Larmor frequenciesfor the higher static magnetic now available at 3 T and above. Becausethe SNR from MR samples increases with the magnet field strength, theability of this coil to resonate and operate efficiently at higherfrequencies means that it can be used at high field strengths to obtainfurther SNR gains. This increase in signal to noise ratio provides asubstantially improved image of the object to be analyzed within theresonator during the operation of the MR system, for example.

The resonator disclosed by Vaughan has proven effective in MR systemsproviding increased SNR, homogeneity, transmit efficiency, fill factor,decreased electric field losses, and higher operational frequencies.However, there are still some problems associated with both theresonator disclosed by Vaughan and the previously known lumped elementresonators.

An ideal clinical head coil for example would lend itself to the easiestand most comfortable subject positioning, leave the subject's faceuncovered once the subject is in position, and would include theseaccess and comfort features without compromising coil performance,safety and reliability. It is preferable for a head coil to be bothcomfortable and accessible for the subject and easy to use for thetechnician. However, it is difficult to provide comfort andaccessibility for the subject and ease of use for the technician andmaintain a high coil performance. The ideal coil should have a removabletop as well, to allow for comfortable subject positioning in the coil.Furthermore, some commercial systems don't provide the space for a coilthat slides over a subject's head. Accordingly, several commercial coildesigns already incorporate this “pop top” feature. However, thesecommercial coils that separate into halves are not popular with someresearch applications such as fMR. Apparently the electrical contactsthat are broken and remade to open and close the coil each time a newsubject is loaded, become unstable over time due to wear and oxidation,resulting in noise “spikes” and temporal instabilities often seen in EPIimages for example. These electrical contacts are required to completethe end ring current paths in birdcages and similar coil structures.While commercial coils must meet rigorous FDA safety criteria, it couldbe imagined that electrical contacts in a coil might possibly posesafety risks in certain situations, especially where electrolytic bodilyfluids were present.

Similarly, an ideal body coil might be as small as possible and fitclose enough to the human trunk for efficient transmission to andreception from a region of interest, but allow room for subject comfortand access. The present body coils are built very large to allow foraccess and comfort, but as a consequence are very inefficient and arepoorly couplet the MR region of interest in a body. RF power amplifiersof 35 kW and more are required to compensate for the inefficiencies of abody coil used in a 3 T magnet for example. Still, these coils providelittle shoulder room for the largest human subjects.

Limb coils, especially leg coils, are similarly limited by conventionalpractice. A leg volume coil for example must be oversized to make roomfor a leg with a foot to be threaded through the cylindrical structure.Or a leg coil has a removable top to provide easier access for a closerfitting, more efficient coil. This latter coil however by conventionaldesigns requires the problematic electrical contacts described for thehead coil above.

Typical existing head only MR systems are one-piece units. A significantproblem with this structure is that many medical subjects do not possessthe physical ability to manipulate their heads and bodies into thepositions required for the MR without significant difficultly or pain.Typically, the subject must try and navigate their head into the smalldiameter of the head only system. This can be painful or impossible formost medical subjects who are asked to do this while lying on theirback.

Because of the inherently low SNR of the MR signal, these signals mustbe acquired and averaged a sufficient number of times to improve the SNRto a significant value. MR data acquisition by signal averaging ishighly intolerant of motion in the MR sample or subject. Accordingly,human subjects are required to remain motionless for the duration of anMR scan, sometimes lasting 30 minutes or more. Movement will result inlower resolution images and in image “ghosting”, thereby limiting thediagnostic quality of an image and often requiring a repeated imagingsession. To minimize head motion for example, MR operators will ofteninsert padding around the subject's head to provide head restraint.While this has the effect of reducing the subject's head movement, itdoes not eliminate all of the subject's head movement. Further, all ofthe padding placed around the subject's head can apply uncomfortablepressure and can intensify the subject's feelings of claustrophobia.Therefore, the purpose behind having a high performance coil with abetter signal to noise ratio is defeated if the subject cannot remainstill.

Another, problem associated with many MR protocols, is they can bepainfully loud. Typically, subjects are given earplugs or headphones tomuffle the noise (in most MR centers the subject can even bring theirown cassette or CD to listen to). The acoustic noise is attributed tothe electro motive forces generated by switched electrical currents inthe wires of the magnet's gradient coils. Stronger magnet fields andstronger or faster gradient current switching generate greater acousticamplitudes. While the methods mentioned above are generally effectivefor gradient noise reduction, coils must be built to larger and lessefficient dimensions to accommodate the head restraint and hearingprotection devices placed about the head.

Visual input/output is often required for a subject receiving an MRexam, for diagnostics or research. These I/O visual devices help tominimize claustrophobia, provide visual cues, and relay information fromthe MR operator. Visual I/O devices are typically mirrors, prisms, oractive displays located above the subject's eyes. A problem withexisting systems of this sort is that 1) they are often fixed inposition which requires that a subject be adjusted to the device, and 2)they typically protrude above the head coil so as to preclude their usein close fitting head only MR systems, and in head gradient inserts usedin whole body MR systems.

It has been shown that back planes on RF coils can function as an RFmirror to extend the uniformity of the coil's transverse RF magneticfield along the rotational or “z” axis of the coil. A back plane, alsoknown as an end cap, can be used in a coil to make a shorter, andtherefore more ergonomic, better shielded, and more electricallyefficient coil for a desired field of view. Conventional cylindricalbirdcage head coil, as mentioned above, typically do not have a backplane. The lack of a back plane together with the inherently shorteraxial field of a birdcage require the birdcage head coil to be longertypically covering the subject's mouth and chin. This increased lengthof the birdcage has many problems. It creates a head coil, which canincrease feelings of claustrophobia for the subject. Once inside of thehead coil the subject's mouth is located immediately in front of theinside coil wall. A subjects breath pushed back into their face by theinside coil wall creates a very uncomfortable/claustrophobic feeling forthe subject. This is an undesirable result since the MR exam may take 20to 90 minutes. Additionally, general medical access and vocalcommunications are impeded with the coil extending over the subject'smouth and chin. Further, if the subject has a large head, nose, and/orchin it becomes increasingly difficult to fit the subject's head insideof the coil. Another additional disadvantage for coils not having endcaps is additional electromagnetic energy is lost from the top of thecoil and thus the coil is less efficient at high frequencies.

While a back wall in a head coil is more desirable for coil performanceand ergonomics in relation to the subject's mouth and chin (i.e., with aback wall the head coil body can be shorter and thus the head coil doesnot have to extend over the chin), a back wall is undesirable for acouple of reasons. First, a back plane closes off one end of the coilgiving the appearance of putting ones head into a bucket rather than anopen cylinder. This can increase a feeling of claustrophobia for thesubject. Second, the back plane limits access to the subject from theback of the magnet. In coils without back planes, leads forphysiological monitors, anesthesia and/or respiratory hoses, EEG leads,communications lines, etc., can be passed. In these systems visualsignal projection is often performed from the rear of a magnet andthrough the back of a coil to mirror or prism systems mounted above thesubjects' eyes. Therefore, presently, head coil manufacturers mustchoose between the benefits of having a coil back plane or end cap orthe benefits associated with access to the subject provided by headcoils with no end cap.

As stated above, a problem associated with head coils is the amount ofspace provided inside of the coil. RF coils transmit MR stimulus to thesubject and receive signals back most efficiently when the coil is asclose as possible to the subject. Therefore, for RF coil performanceconsiderations, space inside a coil should be only enough for subjectcomfort and for the inclusion of devices useful for safety, headstability, and communication with the subject. As stated above, normallya subject must wear earphones or plugs for hearing protection and haveseparate pads inserted around the head to hold the head motionless forthe MR exam. Further, there is typically some sort of visual and/oraudio communications device adjacent to the head so that the MR operatorcan communicate with the subject. However, the padding, hearingprotection and communication equipment can not only make the MRexperience uncomfortable for the subject but this equipment alsooccupies limited space within the head coil.

All of these problems listed above, individually and collectively,degrade the overall quality of NMR images and spectra, add to thediscomfort of the subject, and limit subject access for the physician orresearcher. Therefore, what is clearly needed is a high performanceapparatus, which provides for increased signal to noise ratios andimproved MR image quality, while overcoming the problems discussedabove.

SUMMARY OF THE INVENTION

An magnetic resonance apparatus in embodiments of the invention mayinclude one or more of the following features: (a) a coil having atleast two sections, (b) the at least two sections having a resonantcircuit, (c) the at least two sections being wirelessly coupled ordecoupled, (d) the at least two sections being separable, (e) severalopenings allowing a subject to see and be accessed through the coil, (f)at least one cushioned head restraint, and (g) a subject input/outputdevice providing visual data from in front and behind of the coilrespectively; wherein the input/output device is selected from the groupconsisting of mirrors, prisms, video monitors, LCD devices, and opticalmotion trackers.

A magnetic resonance apparatus in embodiments of the invention mayinclude one or more of the following features: (a) a coil having atleast one head restraint, (b) a means for audio communication to asubject connected to the head restraint, (c) a means for active orpassive protection for a subject's hearing, (d) at least three headrestraints to provide a three-point head restraint, (e) a means tosecure said head restraint to a subject's head, and (f) a head cushion.

A TEM coil providing MR imaging in embodiments of the invention mayinclude one or more of the following features: (a) a coil providing atleast two head restraints, (b) the head restraints having a means foraudio communication to a subject, (c) a means for active or passiveprotection for a subject's hearing, (d) a head cushion to provide athree-point head restraint, (e) a means to secure said head restraintsto a subject's head, and (f) a means to accept an audio signal input.

A magnetic resonance apparatus in embodiments of the invention mayinclude one or more of the following features: (a) a coil having atleast two separable sections, (b) a input/output device, (c) a tracksystem located within an open face area in the coil, and (d) theinput/output device being slideably mounted on said track.

A TEM coil in embodiments of the invention may include one or more ofthe following features: (a) an input/output device, (b) a track systemlocated within an open face area in the coil, (c) the input/outputdevice being slideably mounted on said track, and (d) the input/outputdevice is selected from the group consisting of visual transducers,olfactory transducers, gustatory transducers, or auditory transducers toprovide two way communication or stimulus.

An RF coil for magnetic resonance in embodiments of the invention mayinclude one or more of the following features: (a) the coil comprising aplurality of sections, (b) the plurality of sections being with wirelessconnections and able to wirelessly couple electromagnetic energy betweenthe plurality of sections or the plurality of sections being reactivelydecoupled, (c) the plurality of sections being separable, and (d) aguiding means to assure mutual alignment between said plurality ofsections.

A method of manufacturing a magnetic resonance device in embodiments ofthe invention may include one or more of the following steps: (a)providing a coil having a plurality of sections; the plurality ofsections being separable, (b) attaching a head restraint means toprevent movement of a subject's head, provide communication with thesubject, and protect said subject's hearing, (c) inserting a slideableinput/output device within an open window on said top section to providesubject with two way communication, (d) creating a rear projection slotproviding a channel positioned in a back plane of said coil to provideaccess for a rear visual projection system.

A method of performing magnetic resonance imaging in embodiments of theinvention may include one or more of the following steps: (a) providinga coil having a top section and a bottom section; said top section andbottom section being separable and wirelessly connected, (b) placing asubject within the bottom section, (c) placing the top section inelectromagnetic communication with the bottom section, (d) providing aplurality of openings in the top section to allow a subject to seethrough said coil and be accessed through said coil, (e) providing atleast one cushioned device for head restraint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a construction of a resonator.

FIG. 1 a is a profile view of a construction of a resonator according toFIG. 1.

FIG. 2 is an exploded view of an internal construction of a resonatoraccording to an embodiment of the present invention.

FIG. 2 a is an exploded view of a resonator according to an embodimentof the present invention.

FIG. 2 b is a side profile diagram of a simulated subject's head withina resonator for the present invention.

FIG. 2 c is a profile view of a prior art bird cage coil.

FIG. 2 d is a profile view of a TEM cavity resonator.

FIG. 2 e is a profile view of a resonator according to an embodiment ofthe present invention.

FIG. 2 f is a profile view of a resonator according to an embodiment ofthe present invention.

FIG. 3 is an enlarged view of the exploded view of a resonator as shownin FIG. 2 a.

FIG. 4 is an exploded front profile of a resonator for the presentinvention.

FIG. 5 is a rear side profile view of a resonator for the presentinvention.

FIG. 6 is a front profile of a resonator for the present invention.

FIGS. 7 a, b, c, d compare lumped element resonant circuits totransmission line analogues.

FIGS. 8 a, b diagrammatically illustrate a coaxial cavity used for highfrequency volume MR coils according to the invention.

FIGS. 9 a, b diagrammatically illustrate a tuned TEM resonator accordingto the invention.

FIG. 10 is a diagram of the modes of an eight element TEM resonator.

FIGS. 11 a, b, c, d, e show B₁ flux line representations for the fivemodes of an eight element TEM resonator, according to the invention.

FIG. 12 shows time dependent B₁ magnetic vector equipotentials for mode1 of a head loaded coil according to the invention.

FIGS. 13 a, b show time-dependent B₁ contours for mode 1 of the phantomand head loaded resonator according to the invention.

FIGS. 14 a, b show alternative circuit models for a tuned TEM resonatoraccording to the invention.

FIG. 15 is an elevation, in perspective, of a tuned TEM resonator for ahigh frequency, large volume MR coil, according to the invention.

FIG. 16 is a circuit diagram of a driven coaxial line element used inthe present invention.

FIG. 17 is a diagram of transmission return loss showing resonances of asixteen element resonator according to the invention.

FIG. 18 is an elevation, in perspective, of a dual frequency tunable TEMresonator for a high frequency, large volume MR coil, according to theinvention.

FIG. 19 is a sectional view of a portion of FIG. 18.

DETAILED DESCRIPTION

To assist in an understanding of the invention, a preferred embodimentor embodiments will now be described in detail. Reference will befrequently taken to the figures, which are summarized above. Referencenumerals will be used to indicate certain parts and locations in thefigures. The same reference numerals will be used to indicate the sameparts or locations throughout the figures unless otherwise indicated.

The present invention is not limited to only distributed circuit cavityresonator head coils, and may be employed in many of various types of MRhead coil devices. It is to be further understood, moreover, the presentinvention may be employed in many of various types of MR devices and isnot limited only to head coils. For purposes of illustration only,however, the present invention is below described in the context ofcavity resonator head coils.

With respect to FIGS. 1 & 1 a, a general construction of a cavityresonator is shown. Additional structures and teachings for constructionof a cavity resonator, which can be utilized in the present inventionare discussed in U.S. Pat. Nos. 5,557,247, 5,744,957, 5,886,596, U.S.Provisional Application 60/135,269, U.S. Provisional Application60/222,144, U.S. Provisional 60/378,111, and U.S. Provisional60/373,808, which are herein incorporated by reference in theirentirety. Generally a cavity resonator (coil) 10 is comprised of anelectrical circuit tube/board 12, a front cavity wall component 14, alateral cavity wall component 16, and a back cavity wall component 18all contained within a coil shell 22.

With reference to FIG. 2, an exploded view of an internal constructionof a resonator according to an embodiment of the present invention isshown. Generally coil 24 is comprised of electrical circuit tube/board12 and 12′, front cavity walls 14 & 14′, lateral cavity walls 16 & 16′,and back cavity wall 18 & 18′ all contained within coil shell 22 & 22′.Electrical circuit tubes 12 and 12″ are generally comprised of circuitelements which may be transmission line elements including coaxial line,flat line, stripline, microstrip, wave guide or other distributedelements, inductors, capacitors, PIN diodes, or other lumped elements,printed or etched circuit boards, preamps, TR switches, phase shifters,amplitude modulators, and other electronics devices. Tubes 12 and 12′are inductively coupled by mutual inductance of the current elements ineach. Cavity walls 14, 14′, lateral cavity walls 16, 16′, and backcavity wall 18, 18′ are all electromagnetic shields or conductive stripscomprised of conductive foil to complete the circuit, to limit radiativelosses and to provide a Faraday shield for the coil 24. Coil shells 22,22′ are comprised of a nonconducting packaging material such as plasticor fiberglass to house the internal components of coil 24, which arediscussed above. When combined, front wall 14′, tube 12′, lateral wall16′, back wall 18′, and coil shell 22′ are combined to form top section28. In addition, front wall 14, tube 12, lateral wall 16, back wall 18,and coil shell 22 are combined to form bottom section 26. The internalconstruction of coil 24 is only essential to the present invention tothe extent that it agrees with the external construction as shown inFIGS. 2 a–6 and provides for some preferred manufacturing methods andmaterials. It is contemplated that coil 24 can have most any type ofinternal MR structure, however, preferably resonator 24 has a generalinternal structure similar to that of the teachings of U.S. Pat. Nos.5,557,247, 5,744,957, 5,886,596, U.S. Provisional Application60/135,269, U.S. Provisional Application 60/222,144, U.S. Provisional60/378,111, or U.S. Provisional 60/373,808.

Through research it has been found that hospitals, physicians, and MRsystem operators prefer to have a coil, which comes apart in two halvesfor ease of subject accessibility. With respect to FIG. 2 a, an explodedview of an embodiment for a coil of the present invention is shown. In apreferred embodiment, coil 24 is comprised of a top section 28, and abottom section 26. While FIGS. 2–6 show coil 24 having only a top 28 andbottom section 26, it is fully contemplated that coil 24 could have aplurality of sections without departing from the spirit of theinvention. However, for the purposes of this discussion, coil 24 ischaracterized as having a top 28 and bottom section 26. Generally, sincethe two halves of coil 24 are wirelessly coupled through inductive orcapacitive coupling, there is no need for electrical contacts to bindtop section 28 with bottom section 26. Basically coil 24 is comprised ofconductor elements located in top section 28 mutually coupled toelements in bottom section 26. This reactive coupling is possibleutilizing structures similar to those in U.S. Pat. Nos. 5,557,247,5,744,957, 5,886,596, U.S. Provisional Application 60/135,269, U.S.Provisional Application 60/222,144, U.S. Provisional 60/378,111, or U.S.Provisional 60/373,808. Coil 24 can be viewed as an array of resonantunits (each unit comprised of a conductive element with cavity segment)all reactively coupled to each other. Thus there is no need for hardelectrical contacts to transfer energy between top section 28 and bottomsection 26, thus the energy is transferred wirelessly. Alternately, asshown in FIGS. 2 e, and 2 f, a plurality of sections 112 are reactivelydecoupled and driven by independent transmitters, or received byindependent receivers or both transmit and receive such as is used inparallel imaging. These separate resonant circuits not only make topsection 28 and bottom section 26 separable from one another they alsohave the added benefit of preventing switched gradient current inducededdy currents in coil 24.

With reference to FIG. 2 c, previously known devices such as birdcage100, discussed above, require electrical contacts to complete an endring 102 circuit on each end. Birdcage coil 100 design has one or moreend ring 102 circuits at each end of the coil design, which providecurrent return paths to separate legs that traverse between two rings102. Thus, the operation of a birdcage coil 100 design relies on currentflow through end rings 102. Therefore, separating birdcage 100 into twohalves requires hard electrical contacts between the two halves to bebroken. Electrical contacts provide many problems including being proneto wear, oxidation, intermittency, and potential safety hazards for thesubject.

The current paths on conventional birdcage coils 100 are dependent onend rings 102 making birdcage 100 inductance dependent on the diameterof the coil. Large coils such as head and body coils are very inductiveand therefore resonate at lower frequencies.

With reference to FIGS. 2 d, 2 e, and 2 f, resonant cavity 110 circuitof the present invention does not depend on a current return path on anend ring 102, as does a birdcage 100. The current return in thepreferred embodiment occurs on its associated cavity wall 108 segment,creating an electrically shorter, and therefore less inductive, morecurrent uniform, higher frequency, and more efficient circuit than anequal sized birdcage 100. The operational frequency of birdcage 100 isdependent on and limited by the inductive end ring 102. TEM or cavityresonator 110 whose operational frequency is not dependent on or limitedby an end ring 102 however can obtain much higher frequencies.

The current paths on TEM resonator 110 are not dependent on end rings102, but rather on cavity wall 108 to provide a “return” path forcurrent elements 109. TEM coil 110 can therefore be arbitrarily large indiameter such as a large body coil, and still resonate at frequencieslimited only by the size of an individual line element. Segmented TEMcoils 110 shown in FIGS. 2 e, and 2 f, are shown to highlight theindividual segments 108 or line elements 109 of which a TEM coil 110 iscomposed. Because the operation of TEM coil 110 does not depend on anend ring 102, segments 112 from coil 110 can be removed as shown withoutaffecting the operational coil.

Therefore, the present invention utilizes top section 28 reactivelycoupled, inductively and/or capacitively, to bottom section 26 to allowfor a separable design without the necessity of hard electricalcontacts. The two sections of coil 24 are coupled reactively to oneanother so that all electrical circuits of coil 24 are sealed harmlesslyinside dielectric coil packaging 27. The present invention not onlymakes coil 24 more accessible to subjects, it also minimizes or preventsany electrical shock hazard to the subject. Further, the separation oftop section 28 and bottom section 26 assists in preventing eddycurrents.

Another benefit of the reactive design of coil 24 is the ability tocreate an open window 29 substantially near the top of top section 28.As discussed above, a common problem with present systems is the coilgiving the subject either an increased feeling of claustrophobia or notproviding enough room for subjects with large heads, noses, and/orchins. Since, top section 28 has several openings 25, betweeninductively coupled elements of coil 24, the subject can freely see outof coil 24 and thus the feeling of claustrophobia is significantlyreduced. Additionally, general medical access and vocal communicationsare not impeded due to open sections 29 & 25. Further, if the subjecthas a large head, nose, and/or chin, open window 29 allows the subjectto fit comfortably within coil 24 as is depicted in FIG. 2 b. The use ofopen window 29 allows a larger head to fit, into a smaller, closerfitting, and therefore more efficient coil 24.

FIG. 3 shows an enlarged view of FIG. 2 a. With respect to FIG. 3,combination head restraints 30 and head cupped cushion 31 are shown. Asstated above, regardless of the amount of padding used to prevent motionof the subject's head there is inevitably still some motion. Moreover,the padding can cause the subject some discomfort from compression andan increased feeling of claustrophobia. Combination head restraints 30and head cushion 31 of the present invention not only provide animproved cushioned 3-point head restraint system from an ergonomics'viewpoint, but head restraints 30 also provide communications to thesubject from the MR operator, relaxing music or other entertainment tothe subject, and hearing protection by passive or active soundsuppression means. Audio input into the head restraints 30 may be bycable, optic fiber, sound tube, or wireless means.

As can be seen from FIG. 3, head restraint earpieces 30 can bepositioned in any direction to accommodate the size of the subject'shead. Furthermore, head restraints 30 and head cushion 31 may come in avariety of sizes for use on a variety of subjects. Head restraints 30and head cushion 31 would be chosen, depending on the subject's headsize to ensure that the subject's head is held snugly in coil 24 toprevent motion, but not so snug as to cause pain. Further, because headrestraints 30 and head cushion 31 are compact they do not crowd coil 24,thus allowing the use of a smaller more efficient coil while reducingany feeling of claustrophobia. Other methods of adjusting headrestraints 30 and head cushion 31 are contemplated, such as havingcushions that can expand and contract to the subject's head by simplyrotating them in a clockwise-counterclockwise motion, cams, or othermeans without departing from the spirit of the invention. With furtherreference to FIG. 3, guiding slots 32 are shown. Guiding slots assist inassuring mutual alignment between top section 28 and bottom section 26.

With reference to FIGS. 4 & 5, an I/O device of the present invention isshown. I/O device 40 can be any type of I/O device including but notlimited to mirrors, prisms, video monitors, LCD devices, optical motiontrackers, etc. In one embodiment I/O device 40 has a front mirror 44 anda rear mirror 46 facing the front and rear of coil 24 respectively andis mounted on track 42 over the subject's face. I/O device 40 can thenbe adjusted to multiple positions, directions, and configurationswithout moving the subject. Further, in contrast to prior systems wherethe outside radius of the coil is increased by mounting I/O device(s) ontop of or extending from the top of a coil, I/O device 40 is mountedwholly or in part within the radius or outside dimensions of coil 24. Itcan be seen from FIGS. 4 & 5 that I/O device 40 is particularly wellsuited for mounting inside open window 29 (discussed in more detailabove) above the face in coil 24. I/O device 40 can be moved along track42 from one end of the coil to the other. By sliding I/O device 40 alongtrack 42, it can be adjusted to any subject's useful viewing angle,including looking forward or backward. For example: receiving visualinformation projected to the subject from the front or the back of theMR magnet (discussed in more detail below). The versatile and lowprofile construction of I/O device 40 allows for coil 24 to be used inclose fitting head only MR systems, and in head gradient inserts used inwhole body MR systems. Further, it is contemplated I/O device 40 couldbe replaced or complemented by similar platforms to perform other I/Ofunctions such as positioning temperature, air, oxygen, anesthesia, IVtubes, or the like relative to a subject's mouth or nose. It is furthercontemplated that input/output device 40 can be any visual, olfactory,gustatory, or auditory transducers providing two way communication orstimulus. I/O devices can also be used for delivery of therapy, roboticexecution of surgery, and physiological monitoring includingtemperature, blood pressure, respiration, EEG, EKG, etc.

With respect to FIG. 6, a front view of a resonator is shown. Asdiscussed above, back planes on RF coils function as an RF mirror toextend the uniformity of the coil's transverse RF magnetic field alongthe rotational or “z” axis of the coil. The lack of end rings on a TEMcoil further enhances the field extent on the z-axis. A back plane orend cap can be used in coil 24 located within back wall 62 to make thecoil shorter on the z axis, and therefore more ergonomic, bettershielded, and more electrically efficient coil for a desired field ofview. In coil 24 for example, back wall 62 of coil 24 housing a backplane makes it possible to build a coil short enough to fully expose asubject's mouth, and make the coil more efficient at higher frequencies,while still covering the whole head field of view desired for a standardhead coil. In contrast, a conventional cylindrical birdcage head coilwithout an end cap may be twice as long, less efficient at highfrequencies, and more prone to promoting claustrophobia. A back wall ina head coil is more desirable for coil performance and ergonomics on the“mouth” end of the coil.

Visual signal projection is often performed from the rear of a magnetand through the back of a coil to mirror or prism systems mounted abovethe subjects' eyes. Blocking this access with a coil back plane orendcap therefore prevents visual signal projection. The presentinvention solves the rear magnet access problem; at least in part byproviding a substantial channel 60 in back wall 62 of coil 24. Opening60 is located high in coil 24 to minimally affect the imagingperformance in the head area of coil 24 while giving maximum access forrear visual projection systems. Therefore, opening 60 allows the coil 24to preserve the advantages of a closed end coil, while allowing for mostof the benefits of an open-ended coil. Further, channel 60 providesgeneral medical access for temperature, air, oxygen, anesthesia, IVtubes, EEGleads, physiological monitors, or the like.

The tuned TEM resonator of the present invention is exemplified by atransmission line tuned coaxial cavity resonant in a TEM mode. Thecoaxial line tuning elements correspond to the “slotted” hollow centerconductor of this coaxial cavity.

In FIGS. 8 a and 8 b, the TEM cavity volume coil is shown in the form ofa reentrant cavity 121 such as is used for klystrons and microwavetriodes (8). The cavity is reentrant in that its conductive boundaries122, 123, 124, 125 and 126 extend into or reenter the volume encompassedby the cavity. Using transmission line theory, approximations of thedistributed impedance coefficients R, L, C, the fundamental TEM moderesonant frequency, and the resonant Q of this cavity are made. Thecoaxial cavity is similar to a coaxial line shorted at two ends andjoined in the center by a ring capacitor C_(c)′ and having thedimensionso.d.=2b_(c)′i.d.=2a_(c) length≈21.

The input impedance to each low loss, shorted coaxial half cavity isgiven byZ _(in) =Zo tan h(α+jβ)l  (1)

The characteristic impedance for this coaxial cavity is derived from Eq.(2).Z _(o) =√L/C=(η/2π)(ln b/a) for η=√(μ/ε)  (2)

For a coaxial cavity whose outside radius is b_(c) and whose insideradius is a_(c)′Z _(oc)=(η₀/2π)ln(b_(c) /a _(c))  (3)Z _(inc) =Z _(0c) tan h(α_(c) +jβ _(c))l, α _(c) ≈R _(c)/2Z _(0c)  (4)for α=0,Z _(inc) =Z _(Oc) tan β_(c)1  (5)

For Z_(inc)=X_(inc) Eq. (12), the distributed inductance L_(c) of thelow loss cavity is:L _(c)=2X _(inc)/ω2Z _(0c) tan h(α_(c) +jβc)l/ω  (6)

For the low loss coaxial cavity,L _(c)=2Z _(0c) tan (β₀ l)/ω=2Z _(0c) l/υ ₀  (7)

Compared to a lumped element coil circuit enclosing a given volume, theinductance Lc of the coaxial cavity containing the same volume issignificantly lower. The center gap series capacitance Cc required toresonate this cavity at the design frequency f=ω/2π is:C _(c)=½ωX _(inc)=½ω(Z ₀ tan h(α_(c) =jβ _(c))1)  (8)

And for the low loss approximation,C _(c)=1/(2ωZ ₀ tan β₀ l)  (9)

This center conductor gap capacitance could be supplied in lumpedelements or by a capacitive ring 127 as pictured in FIGS. 8 a and 8 b.Stray capacitance Cs between the inner and outer walls of the coaxialcavity contributes a small fraction of the value Cc. Cs ultimatelylimits the cavity's self resonance frequency which is substantiallyhigher than that of the lumped element coil of the same volume; a cavityof human head coil size will resonate above 500 MHz, for example. Thestray capacitance for the lossless coaxial cavity is approximated by,C _(s) =πεl/ln(a/b)  (10)

The fundamental TEM mode resonant frequency f₀ of the cavity given byEqs. (9, 10) is:f ₀=1/(2π√(L _(c) C _(c)))  (11)

The series conductor resistance R_(c) in the cavity is determined by thefrequency dependent surface resistance R_(s):R _(c)=(R _(s)/2π_(c))(1/a _(c)+1/b _(c))21,whereR _(s)=√(ωμ₀/2σ)=1σδ  (12)

Shallow skin depth δ of current flux in a good conductor σ=δ(10⁷), S/mmakes for the requirement of a large surface area of cavities forminimizing R. The skin depth, δ, of copper at 100 MHz, for example, is0.0066 mm.δ=1/√(πfμmuσ)  (13)

The small, high frequency skin depth dimension however allows the use ofvery thin foil or surface deposited conductors which adequately conduct(and contain) RF currents and fields, but effectively attenuate lowfrequency eddy current propagation as induced by switching B₀ fieldgradient currents in the MR application. The characteristically highresonant Q_(rc) of the cavity is:Q _(rc) =βc/2αc=2πf ₀ L _(c) /R _(c)=2πf ₀ Z ₀ /R _(c)−_(c)  (14)

Although the optimum TEM mode Q occurring for the b/a ratio of 3:6 isnot readily achievable in head and body coil applications in the meterbore magnet, practical coil dimensions allow for unloaded Q values inexcess of 1000. The advantageous properties of decreased inductance,decreased resistance, increased frequency, high Q, and serf shieldingfor the coaxial cavity should now be clear.

To permit TEM mode magnetic field propagation in the utility center ofthe coaxial cavity, the hollow center conductor (reentrant wall withcapacitive cylinder), must be slotted (1, 2). Unshielded lumped elementcapacitors or capacitive plates bridging the cavity's slotted centerconductor “leak” to the conservative electric (E) field largely storedwithin these capacitors. Problematic stray E fields, however, whichadversely affect the coil's tune, match, and phase stability as well asefficiency, can be better contained by using N coaxial line elements astubular, shielded capacitors 129 (3), as in FIG. 9 b. The conservative Efield for the resulting TEM resonant cavity is thus largely containedwithin the dielectric regions between the floating outer conductors 130of the coaxial elements, and the reentrant center conductors 131 whichare interrupted or split at 132, but are otherwise continuous with thecavity 121. (FIG. 9 b) The coaxial tuning element used is similar to apair of open ended coaxial lines joined in the center by a continuousouter conductor.

For the fundamental TEM mode resonance, each mirrored pair coaxialelement is in balanced series resonance with the cavity. The currentwaveform peak amplitude is centered on these balanced elements, andeffects a transverse virtual ground plane which bisects the tunedcavity. A desired transverse B₁ field maximum and an E field minimum arethereby generated within the cavity center as desired.

The TEM cavity resonator is tuned to the design frequency by adjustingthe reactance (both L and C) of the line elements 9. Gross adjustment ismanaged by dimensional design of the elements and the cavity. Finetuning is performed by manipulating the center conductor gaps in theelements, i.e. positioning the center conductors to appropriateinsertion depths.

The transmission line element tuned coaxial cavity according to theinvention is the basis for high frequency, large dimensioned volumecoils for MR applications, and can be briefly characterized as a “tunedTEM resonator”.

Transmission line theory (7) provides the design equations for the TEMresonator. The input impedance to each open coaxial half element isgiven by Eq. (15)Z _(in) =Z ₀ cot h(α=jβ)l  (15)and its characteristic impedance is derived from Eq. (2). For the inputimpedance Z_(in)=X_(ine) and characteristic impedance Z₀=Z_(0e)′ thedistributed capacitance C_(e) for each of the coaxial tuning elementsis:C _(e1/2=ω) X _(ine)=1/(2ωZ _(0e) cot h(αe+jβe)1)  (16)

The distributed capacitance of a series pair of lossless open linesegments is easily calculated using jZ₀ cot β1 for approximating CeCe=tan βe½ωZ _(0e)≈½Z _(0e)υ=πε_(e)1/ln(b _(e) /a _(e))  (17)

A coaxial tuning element of the cavity length 21, FIG. 9 b, isconstructed for a desired C_(e) by choosing the appropriate dimensionsfor the center conductor radius a_(e) and the outer conductor radiusb_(e). The dielectric material for the elements is typically not air,therefore ε_(e)=ε_(r)ε0 where the relative permitivity for commonly usedTeflon is ε_(r)=2. The distributed inductance L_(e) for each coaxialelement can be similarly derived from Eq. (15) for L_(e)=2X_(ine)/ω, andapproximated by:L _(e)≈(Z _(0e)21e/υ)=Z _(0e)√(μ₀ε_(e))21  (18)

From Eq. (12), the resistance R_(e) per element is:R _(e)=((1/δσ)/2π) (1/a _(e)+1/b _(e))21  (19)

The total series inductance L_(t)′ capacitance C_(t)′ and resistanceR_(t)′ for an N element tuned TEM resonator are:L _(t) ≈L _(c) +L _(e) /N  (20)C_(t)≈NC_(e)(21)R _(t) ≈R _(c) +R _(e) /N  (22)

Resonant frequences are:f _(r)=ω_(r)/2π=ευ/2π=nυ/41=n/(41√(LC)), n=integer  (23)

In the approximations for L and C above, small amounts of mutualinductance and stray capacitance in the TEM resonator structure were notconsidered. By Eqs. (20,21, and 23) the TEM mode resonance for the tunedTEM resonator is:f _(0t)≈1/(2π.pi√(L _(t)C_(t)))  (24)

The Q factor for the TEM resonator is:Q_(t)≈2πf ₀ L _(t) /R _(t)  (25)

When coupled inductively or capacitively to a matched transceivernetwork, the quality factor becomes Q/2 for the circuit loaded TEMresonator.

TEM resonator modes: The tuned TEM resonator is a closed-ring periodicdelay line similar to a traveling-wave magnetron oscillator (9).

In the traveling wave type oscillation, the mode M dependent phasedifference ΦM between the electrical oscillations of N successive tuningelements is such to produce a rotating AC field or traveling waveperiodic with ^(τ) _(M) in the interaction space between the elementsand the cavity of the resonator.Φ_(M=)2πM/N=β ₀ ^(τ) _(M)  (26)

The traveling wave propagates in the azimuthal direction at an angularphase velocity ω_(M) for the fundamental harmonic of mode M and phaseconstant β₀ where angular phase velocity ω_(M) equals the resonant oreigen frequency of the corresponding mode.+ω_(M)32+β₀ dΦ/dt  (27)

In the pass-band of the resonator, +Φ<π, therefore from Eq. (27),0<M<N/2 for the integer M. (N/2+1) resonant modes are possible in thetuned TEM resonator.M=NΦ _(M)/2π  (28)

The fundamental modes and corresponding resonant frequencies of theeight element tuned TEM resonator are graphically described in FIG. 10.The ordinate y is the unitless “retardation” ratio of the free spacepropagation velocity to the “slow-wave” angular velocity of the closeddelay line resonator of radius a.y=c/aω=Mλ ₀/2πa=Mλ ₀ /Nτ  (29)

The abscissa is the ratio of the free space wavelength λ₀ to the modalperiod τ_(M)=2πa/M for the resonator. The curves y=f(λ₀/τ) for constantM/N are constant lines through the origin. The frequencies of thedifferent fundamental modes are determined by the intersections of theseconstant mode lines with the dispersion curve:c/aω=f(λ₀/τ)  (30)

Because the angular phase velocity ω has positive and negativecomponents (traveling waves propagate in two directions around theclosed-ring resonator), separate dispersion curves of positive andnegative phase Φ may exist resulting in more possible frequencies forthe tuned TEM resonator, N in total.

The lowest frequency corresponding to M=0 (the cyclotron frequency mode)is the self resonance of the tuned TEM resonator Eq. (24). For thisfrequency with all tuning elements oscillating in phase, Φ=0°. Thehighest frequency and upper boundary for the fundamental modescorresponds to M=N/2, the π mode. In this mode all tuning elements arein phase opposition with adjacent elements, Φ=+180°. A single frequencystanding wave results.

The remaining (N/2−1) modes of resonance are degenerate doublets forimperfect TEM resonator symmetries. A slight departure from circularsymmetry in the resonator will cause + dispersion curve separationresulting in degenerate mode splitting. The azimuthal currentdistribution as a function of Φ in the resonator elements can begeneralized for an imperfect coil as a Fourier series of positive phase(co-rotating) and negative phase (counter-rotating) traveling waves ofunequal amplitude and phase.I=Σ _(I) ^(∞)(A _(M) ^(cos)(ωt−MΦ+δ)+B _(M) ^(cos)(ωt+MΦ+ξ))  (31)where δ and ξ are arbitrary phase constants. For perfect circularsymmetry where A=B, and δ=ξ, the doublets converge to a single frequencyfor each respective mode. As coupling between the tuning elementsdecreases, the modal resonances converge toward a single frequencyapproximated by Eq. (24).

Mode M=1 corresponding to Φ=2π/N is the TEM mode of choice for theclinical MR application. This mode produces a transverse B₁ field whosemaximum magnitude and homogeneity are coincident with the centraltransverse virtual ground plane of the tuned TEM volume coil. The 2π/Nmode can be driven in quadrature for improved MR coil transmission andreception efficiency. This M=1 mode is closest to the single cyclotronmode (M=0), and is easily identified in non optimized coils as thelowest frequency split resonance.

According to the invention, if only eight elements of the resonator aretuned for a given frequency, the other eight are tuned for a differentfrequency, i.e. the TEM resonator can be double tuned by tuning even andodd elements respectively to two widely different frequencies (6). Tworesonance groups then result of (N/4+1) modes each. Each resonance groupconsists of 2 single resonances separated by (N/4+1) degenerate doubleresonances.

The second mode of each group generates the desired transverse B₁ fieldfor the MR application. The double tuned TEM resonator so described issimilar to the “rising-sun” magnetron oscillator (9).

These are shown in FIGS. 12 and 18. Since FIG. 18 in effect containsFIG. 12, the following description of my invention will be mainly interms of FIG. 18.

According to my invention, the same coil structure can allow operationat two or more frequencies. For example, the FIG. 12 coil can providefields at either of the commonly used values 70 MHz and 178 MHz. Thatis, while the 16 elements of FIG. 12, as a single set, can be tuned sothat the coil can operate well at any single frequency in thecorresponding range, e.g., (70–200)MHz, the elements can also be tunedso that the coil can operate well at either of 71 MHz and 175 MHz, orother frequency pair in that range. Thus, 8 of the 16 elements,preferably, every other one, can be adjusted to provide operation at 71MHz, and the remaining 8 can be adjusted to provide operation 175 MHz.Such adjustments do not interact, because no element of one set isclosely enough coupled to an element of the other set, that adjustmentof the impedance of an element of the said one set, can significantlyeffect impedance in the said other set.

Of course, in providing multiple frequency operation, it is necessary toprovide different values for other parameters, such as B₁. However, thecoil of my invention can accommodate such different values by havingcorresponding sets of elements, which sets, once tuned, provide thedesired frequencies without further adjustment of any elements, andwithout alteration of the physical configuration of the coil itself. Ineffect, then the coil of my invention is as many coils as there arefrequencies for which the elements of my coil are tuned.

As will be seen from FIG. 18, my multiple tuned TEM resonator design,like the single frequency form of FIG. 12, uses 16 elements. However,these are divided into two groups of 8, each of which is mounted foradjustment independent of the other, by adjusting mechanism shortly tobe described.

In practice, each group is tuned to some desired frequency byappropriately varying the depths of insertion of the center conductors,thereby fixing the elements's distributed impedances.

In use, one set or the other provides the desired field, and ifnecessary, is fine-tuned, just as if only 8 of its 16 elements arepresent. More generally, if there are n elements in all, then m thereofcan form a set, thereby leaving (n−m) elements from which to form one ormore additional sets.

Indeed, one will be able, in general, to provide k tunings, where k, thenumber of sets can be greater than 2.

Turning to FIG. 18, cavity 141, corresponding to cavity 121. FIG. 9 a,is essentially a plastic cylinder 142 having a circular plate 144 and anend ring 145 fixed thereto, these three elements being provided with aconductive foil on its inside, so as to provide the conductive cavityconfiguration indicated in FIGS. 9 a and 9 b. Ring 24 allows a bodymember, such as the human head, to be inserted into the cavity.

Like cavity 121, the cavity 141 is provided with transmission lineelements 149, like elements 9, but being 16 in number. In order toprovide more than one tunable frequency, two, say, for simplifyingillustration of the present invention, a pair of circular non-conductiveplastic plates 146 and 147 have the center conductors 150 of theelements 149 affixed thereto. In this case, every other conductor 30 isaffixed to just one plate. Thus, plate 146 is affixed to the centerconductor 30 of a set of elements 149 corresponding to frequency A, andplate 147 is affixed to the center conductors of the remaining elements149, which correspond to frequency B. The outer conductors 151 of all 16elements 149 are fixed to plate 144 and end ring 145, and areelectrically continuous with the metal foil on cylinder 142, plate 144,and end ring 145.

The conductors 150 are fixed in position by collet clamps 152, whichreleasably secure the center conductors 150. Clamps 152 themselves arefixed in place, as by being embedded in the respective plates, which areshown as transparent, though they could as well be opaque.

It will be evident that during resonator assembly, the depth ofinsertion of conductors 150 in the segments 29 can be set by looseningthe collet clamps, then individually adjusting the depths of insertionof conductors 150 until by actual measurement a resonant conditionexists when RF energy of the desired frequency is used to energize thecoil. This tuning is coarse, but at this point, the collet clamps areset to fix the depth of insertion of conductors 150. However, plate 147can be translated along its axis (which is also the axis of cavity 141),in order to move all 8 conductors simultaneously, so as to vary theirdepth of insertion, by equal amounts.

(This may be taken as a description of the construction and operation ofthe single frequency resonator of FIG. 12. That is to say, in FIG. 12,the only difference is that the 16 center conductors 150 have theirdepths of insertion adjusted, just as in FIG. 18, 8 conductors are soadjusted, in order to tune to the desired frequency.)

The remaining 8 center conductors in FIG. 18 are fixed to the plate 147by collets 32, at their outer conductors 151. As with their 8 fellows,their depths of insertion are adjusted also, but so as to tune the coilto a different desired frequency. As before, collet clamps 152releasably fix the conductors 150 in a circular plate 147 again oftransparent plastic. Like plate 146, plate 147 can be translated alongthe cavity axis, thereby to vary the depths of insertion of thecorresponding 8 conductors simultaneously, and by equal amounts.

Various mechanical movements cam be used for translating the plates 146and 147, and indeed they can be moved directly by hand since theconductors 150 can have something of a friction fit in the dielectric ofthe elements, and in any event, the plates can be clamped in place byobvious means, not shown.

Preferably, I provide a simple screw mechanism, which acts colinearly ofthe plates' axis. Due to the symmetry of the arrangement, the platescannot cock, and screw threading inherently provides clamping.

In FIG. 12, such screw mechanism 153 is just that: a screw 154. Thescrew 154 has its outer end terminated by a knob 155 for turning it, andhas its inner end journaled at 156 in plate 144. The journal 156 may beany suitable hardware rotatably securing the end of screw 154 in plate144, so that when knob 155 is turned, the screw 154 will rotate but willbe prevented from translating along the coil axis.

Screw 154 passes through plate 157, which corresponds to plate 146, viaa threaded bore (not shown) in the plate, the bore being coaxial withthe coil axis. Turning knob 155 therefore causes the plate 157 totranslate along the coil axis, whereby to adjust the central conductors'position simultaneously and by equal amounts.

In FIG. 18, a somewhat more complex screw mechanism 158 is provided.Mechanism 158 has a knob 15, corresponding to knob 155, in that turningit translates plate 146, but as will be seen later differs somewhat,structurally, from screw 153, though otherwise performing the sametuning function.

Corresponding to journal 156, mechanism 158 has journal 160 rotatablysecuring it to plate 144. Screw mechanism 158 is also threaded to plate147 for translating it along the cavity axis.

FIG. 19 shows screw mechanism 158 in longitudinal section. Thus,corresponding to screw 154 and journal 160 of FIG. 12, in FIG. 19, ascrew 161, terminating in the knob 159 at one end is journaled in plate144 for rotation, without translation along the axis of plate 144. Asshown, screw 161 is threaded into plate 146. A sleeve screw 163, inwhich screw 161 freely rotates has at one end a journaled portion 44securing it to plate 146 for rotation without translation when knob 165,which terminates the other end of sleeve screw 163, is turned. Thesleeve screw 163 is threaded to plate 147, so rotation of the knob 165therefore causes plate 147 to translate along the axis of cavity 141.

In use, one first tunes the elements 149 whose center conductors areclamped in the collets of plate 146. Then one turns the other elementswhose center conductors are clamped in the collets of plate 147. Sincethere is negligible coupling among the elements, neither turning affectsthe other.

The tuning features described above contemplate that, at the other endsof the transmission line segments, the initial depth of insertion of thecorresponding center conductors, established in assembling the cavity,will not be changed in subsequent use. (The ring 145 will, of course,support the other outer ends of the elements 149, like the ring 24does.) Additional tuning effect could be had by varying the depth ofcenter conductor insertion at the other ends of the elements 149. Thiscould be managed by an arrangement of rings corresponding to plates 146and 147, which would serve to bodily adjust such depth of insertion, asdo the plates 146 and 147. Note that one end of the cavity needs to beopen for insertion of a body or body member.

In sum, then, one set or the other provides the desired field, and ifnecessary, is fine-tuned just as if only 8 of its 16 elements ispresent. More generally, if there are n elements in all, then m thereofcan form a set, thereby leaving (n−m) elements from which to form one ormore additional sets.

Indeed, one will be able, in general, to provide k tunings, whereby k,the number of sets, can be greater than 2.

The same approach allows the coil to be multiply tuned, in general,i.e., to three or more resonances.

TEM resonator B₁ fields: The free space magnetic vector potentialcontours (flux lines) for the five modes of the eight element TEMresonator are schematized in FIGS. 11 a–11 e. The mode M=1 is theobvious choice for efficient, homogeneous magnetic coupling to a centralregion of interest. Such free space AC fields are often approximated bythe DC field Biot-Savart law. At radio frequencies beginning at about100 MHz however, static field approximations of RF fields in humananatomy are no longer accurate. Similarly, simple homogeneous, isotropicgeometries (spheres and cylinders) are not appropriate for modeling thehuman load.

Viewing the human body as a heterogeneous, lossy dielectric of tissuewavelength proportions, the electromagnetic propagation boundary effectsof refraction, reflection and attenuation must be considered.Substantial axial eddy current shielding and orthogonal displacementcurrent extension of the B₁ field are observed in human tissues at highfrequencies. Fully time-dependent equations and complex numerical modelsare required for describing the high frequency coil B₁ fielddistribution in anatomic regions of interest. A time-harmonic magneticfield B₁/μ in a lossy, anisotropic, inhomogeneous coil-tissue system canbe described by the differential form of the Maxwell-Ampere Law (10):∇X B ₁ /μ=J _(c) +δD/δt  (32)

By Ohm's Law the current density J_(c)=σE, and by Euler's Law theelectric field displacement δD/δt=δεE/δt=j ωεE so that Eq. (32) can berewritten as:∇X B ₁/μ=(σ+jωε)E  (33)

The complex value of E can be written in terms of the magnetic vectorpotential A, and the electric scalar potential Ψ, such that:∇X B ₁/μ=(σ+jωε.)(−jωA−∇ψ)  (34)

Influencing the B₁ distribution and loss in human tissues adjacent tothe coil are the B₁ field induced eddy current density, J_(e)=−jωσA, andthe accompanying electric field displacement current density,J_(d)=−jωεE=ω²εA for tissue specific values of σ and ε. The magneticvector potential lines A, and the contours B₁=∇X A generated in a humanhead model by specified resonator element currents can be determined bysolving numerically for A and Ψ in the equation:∇X 1/μ∇X A=(σ+jωε)(−jωA−∇ψ  (35)

In FIG. 12, the Finite Element Method (FEM) is used to model a humanhead loaded 16 element TEM resonator operating at 175 MHz. The magneticvector equi-potential lines A/m, for mode M=1 of the TEM resonator werenumerically solved by Eq. (35). The two layers of the head were tracedfrom a 4.1 T MR image, and represent brain and scalp/fat/skullrespectively. Table values for frequency dependent σ,ε, and μ of humanbrain and skull/fat were assigned to the appropriate tissue layers ofthe model. The cross sectional view corresponding to the centraltransverse plane of the volume coil, is the Cartesian xy plane for themodel with z being infinite. The coils 16 RF current elements arecontained within a cavity's usual grounded conductive cylinder. Withequal RF current amplitude in the elements separated by phase Φ=2π/N, atransverse electromagnetic (TEM) field is generated within thetransmission line model as shown. Note the flux line rotations presentin the high frequency coil-head model of FIG. 12, but not seen in thefree space/static field representation of M=1 in FIG. 11.

Calculated B₁ contours, T/m for phantom and head loaded TEM resonatormodels are shown in FIGS. 13 a and 13 b. Central to the current elementsin FIG. 13 a, is a 20 cm human head phantom identified by the contourpattern of five concentric circles. The phantom was assigned σ, ε, and μvalues of human brain. All space not occupied by the coil cylinder,elements or subject's head is free. With a 175 MHz, 2 ampere currentassigned to the elements, a 10 μT B₁ field is generated in free space atthe surface of the head phantom. At 175 MHz the head becomes a lossydielectric resonator with a B₁ field distribution substantiallydifferent than that of free space. The model predicts an attenuation ofthe B₁ field by 3 μT (30%) in the outside ⅓ radius of the phantom. Thisis the expected result of eddy current shielding. The model alsopredicts more than a 1 μT (10%) B₁ field enhancement in the inside 1/10radius of the head. This displacement current induced B₁ enhancementcompensates for the increased shielding expected for higher frequenciessuch that B₁ homogeneity is adequate for imaging the human head and bodyin fields of 4 T and higher. FIG. 13 b presents the FIG. 12 scale modelof a head within a coil which is excited with current levels requiredfor gradient echo imaging at 175 MHz. The model predicts

B₁ contours of highest magnitude to be in the frontal and occipitalregions of the head due to their close proximity to the coil elements.The more distant temporal regions are lower in B₁ magnitude. The centerof the brain shows the displacement current B₁ enhancementcharacteristic of the fortuitous coincidence of tissue σ/ε and humanhead electrical dimensions at higher frequencies. Also beneficial is theeddy current retarding effect of the heterogeneity of human tissue andanatomical structure. The more heterogeneous two layer model in FIG. 8 bpredicts a B₁ inhomogeneity of less than +10% magnitude variation overthe head compared to +15% variation over the single layer isotropicmodel of FIG. 8 a. Empirical observations support the predictions fromthe coil-head, and coil-phantom models above (2, 4, 5, 13, 14).

Alternative TEM resonator models: So far, transmission line theory wasused to describe the tuned TEM resonator as a transmission line tunedcoaxial cavity resonator. Alternatively, the TEM resonator can beapproximated as a balanced comb-line, band-pass filter using the lumpedelement circuit of FIG. 14 a. The lumped elements in this circuitapproximate the distributed element coefficients of the transmissionline circuit. Analysis of this lumped element filter circuit modeladhering to methods in the literature for “bird-cage” resonators givesinaccurate results (15, 16). My invention's more accurate approachconsiders the lumped element filter's distributed stripline analogue inFIG. 14 b. This network is a quarter wave (as in FIGS. 7 a and 7 c)comb-line filter interfaced with its mirrored image at the virtualground plane of symmetry indicated by the dotted line. Each coaxialelement, due to its split central conductor, therefore is a resonanthalf wave line (mirrored quarter wave pair, as in FIGS. 7 b and 7 c wavepair) whose bisected center conductor 11 is grounded at both ends to acavity. The elements 9 are coupled via the TEM slow wave propagation hthe cavity. The performance characteristics of this distributedstructure are calculated from TEM assumptions (17).

Methods and Materials

TEM resonator construction: Single and double tuned TEM resonators havebeen built for clinical MR applications from 70 to 175 MHz. Prototypeshave been bench tested to 500 MHz. Thus, FIG. 15 shows a 175 MHz (4.1 T)head coil. Here the tuned TEM human head sized resonator measures 34 cmo.d. (cavity) by 27 cm i.d. (elements) by 22 cm in length. The i.d. andlength were determined by head size while the o.d. was chosen forcompactness with some sacrifice of the optimal coil Q predicted for alarger cavity Eq. (14). The cavity itself is constructed of 1 mil copperfoil 13 fitted to the inside and outside surfaces of an acrylic cylinderEq. (13). Acrylic end members support the reentrant tuning elements andthe copper foil making their continuity with the cavity. The cavitycould be extended with foil on plate 144.

N.B. A “transmission line element” need be no more than a pair, or more,of conductors side by side, and AC coupled to each other by a dielectrictherebetween. It is evident, therefore, that “coaxial cable” is not theonly form the “elements” or “segments” may take in the process of myinvention.

The tuning elements are 21 cm coaxial line segments 9 whose outer member31 is a copper conductor whose i.d. is 12.7 mm (0.5″). and whose centerconductor 30 is a bisected 6.5 mm (0.25″) o.d. copper rod. Teflon sleeveinserts (not shown) provide the dielectric between conductors 150 and31, and are machined to serve as both a dielectric and bearing surfacebetween the two conductors. The Teflon sleeve thickness (b_(e)−a_(e))should be greater than 3 mm to preclude the possibility of dielectricbreakdown and consequential arcing during high peak power transmitting.5 mm Teflon spacers 15 at each end of the coax segment maintainelectrical isolation between the outer conductor 11 of the element 9 andthe cavity foil 13 to which the center conductor is attached. See FIGS.7 and 8 of Röschman (3) for an exemplary element construction of thissort.

The element component diameters and the number of the tuning elementsused are determined for a desired frequency and coil size by startingwith Eq. (25) and working backwards solving for N, a_(e) , and b_(e).Assuming the lossless case for the lines and the cavity simplifies thecalculations required. Using 4N tuning elements in the designfacilitates quadrature drive. Typical values for N are 12 to 16 in ahuman head coil and 24 to 32 in a human body coil. Homogeneity isproportional to N whereas frequency is inversely proportional.

The divided center conductors of the tuning elements are conductivelyconnected to the cavity 121 on one end thereof by the collet clamps 152,and on the other end by copper beryllium spring gaskets, copper straps,or the like. The collet clamps allow for fixed adjustment of theinsertion depths of the center conductor halves during coil fabrication.The gaskets allow for variable adjustment during coil operation. Byvarying the insertion depth of their center conductors, the coaxial lineelements are tuned to a desired coincidence of mode and frequency.

As previously described, the center conductors on one end of the cavityare mechanically linked by a mobile plate and screw mechanism such thatby turning a knob or knobs all can be adjusted in concert to tune theresonator without effecting its field symmetry. Two line elements 9separated by a 90° azimuth angle are connected to a pair of 90° phasedports 21 of a strip line quadrature hybrid for quadrature drive of theTEM resonator. See FIG. 16 which shows one of two driven line elementsfor a quadrature excited resonator.

The hybrid porks are properly phased and impedance matched to the coiland its human head or body load via the reactance of variable reactorscomprising variable series capacitance and/or balun circuits, not shownin FIG. 15, which, however, does show knobs 166 for varying saidreactance. FIG. 18 requires an additional circuit of this sort, so knobs167 would provide for varying reactance. The quadrature hybrids forfrequencies A and B are identified by reference numerals 168 and 169.Reference numeral 170 identifies FIG. 15's quadrature hybrid.

TEM resonator optimization: Frequency tuning, impedance matching,quadrature phase isolation, B₁ homogeneity and sensitivity, and Q areamong the more important performance characteristics to be optimized forany quadrature volume coil. After the TEM resonator is constructed, thefirst step toward optimization is to adjust all elements (2N halfelements) to the equal capacitance values which tone the resonator's M=1mode resonance to the design frequency. An RCL meter is used for elementcapacitance measurements. A network analyzer frequency swept reflectionmeasurement is used to produce the return loss plot identifying the 16element head coil's 9 resonant modes in FIG. 17. Mode 1, located at174.575 MHz will initially appear as a double resonance as mode 2 forthe non optimized coil. By an iterative method of free tuning thecapacitances of individual elements (not unlike the tuning of a bicyclewheel through spoke adjustment), the mode 1 doublet will collapse intothe well matched, high Q, single frequency point as shown. For thequadrature coil, the plot of FIG. 17 must be reproduced for mode 1 ateach of the two drive points independently. Additionally, a transmissiontest should confirm better than 40 dB isolation between the quadraturedrive points, at the design frequency. Maximum B₁ field amplitude shouldbe verified on the axes orthogonal to the bisecting longitudinal planesof the coil containing the driven elements. B₁ transmission fieldmagnitude and homogeneity are measured on the quadrature x and y axeswith a directional magnetic field probe receiver. After the alignment ofquadrature, well matched and isolated, high

B₁ amplitude resonances at the mode 1 design frequency in the unloadedcoil, the optimization process is repeated for the phantom loaded coil.The phantom electrically mimics a human head in volume and conductivity.The resonator is then fine tuned on human subjects. A striplinequadrature hybrid is finally connected to the two capacitively matched,quadrature drive points of the resonator. In the optimized coil, twosuperposed quadrature B₁ fields corresponding to the mode 1 frequencyare measured, with peak amplitudes of 3 dB less gain than the previouslymeasured single drive point linear fields. With all adjustmentscomplete, the sliding center conductors of the coax segments are clampedinto position at both the fixed ring and mobile plate ends of the coil.In the magnet, the resonator is tuned to different loads by turning thecentral knob 155 shown in FIG. 15, 46 are adjusted for precise impedancematching and 90° phase isolation between the two coil ports of thequadrature hybrid. Tune, match, and phase adjustments can readily beperformed on each study.

Results

Single and double tuned TEM resonators perform efficiently from 70 to175 MHz in human head and body applications. The Q ratio of a 175 MHztuned TEM head coil design is a high 840/90 typical of resonantcavities. The tuning range for this tuned cavity design is arbitrarilylarge to an octave or better. This range facilitates the use of a givencoil at different field strengths and for different nuclei. For a wholehuman head, 90° optimization is typically achieved with a 1 kW, 250 μsecsquare pulse. For each MR study performed, the TEM resonator is tuned tofrequency, matched to a VSWR of 1.0 at both coil drive points, andquadrature phase isolated to greater than 50 dB, all without adverselyinfluencing B₁ field symmetry. This In Situ optimization requires threeexternal adjustments and less than three minutes time.

B₁ field patterns observed in phantom and human loads are consistentwith the model predictions of FIG. 13. Equal element currents correlateto maximum homogeneity of B₁ field distribution in axis symmetricphantoms coaxially located within the coil. Head images from resonatorswith equal element currents do not show the best achievable homogeneity.The B₁ contour pattern calculated for equal currents in FIG. 13 predictsthe inhomogenities in the head image of FIG. 18. B₁ coupling of thehighest magnitude is in the occipital region of the head due to thehead's close proximity to the coil elements. The intensity of B₁coupling to the frontal region of the brain which is spaced further fromthe coil elements to allow for the nose, is less. The more distanttemporal regions show the least signal intensity and corresponding B₁field strength. The center of the brain shows the displacement currentB₁ enhancement predicted. At high frequencies, a more homogeneous B₁distribution in the head or other loads lacking perfect axial symmetryand position can be achieved by asymmetrically adjusting individualelement currents while otherwise maintaining optimized performancecharacteristics for mode 1 of the coil. Element currents affectinghigher homogeneity for a design and application can be calculated asbefore by numerically solving Maxwell's equations for finite-elementscale models of the loaded coil. Homogeneity in the human head of 10%variation over a 15 cm d.s.v. has been achieved in practice, using a T1weighted gradient echo sequence wherein TR=2500 ms, TE=17.5 ms, andTIR=1000 ms for a 3 mm, 512² resolution single acquisition image. Only arelatively low level 400 W peak RF prefocussed pulse excitation waveform was required for head images. This is in contrast to theexceedingly high levels formerly predicted in the literature for highfield clinical imaging. High resolution definition of the transcranialvascularity is thought to be enhanced by the high field susceptibilitygradient imparted by the relative paramagnetism of deoxygenatedhemoglobin in venous blood. 3 mm and 5 mm slice thickness brain imagesof 512² and 1024² resolution showing submillimeter detail of brainvasculature and neuroanatomy have been obtained (4, 18). Excellentquality human head and body images have been shown (19).

Using the tuned TEM resonator at 4.1 Tesla, the potential for scientificstudies and clinical diagnostics from anatomic, spectroscopic, andfunctional imaging of the brain has been convincingly demonstrated. With400 μM in plane resolution from 3 mm slices, clinically importantstructures such as the basal ganglia, red nuclei, thalami and thesubstantial nigra were clearly visualized in 20 volunteers, potentiatingimage based diagnoses of neurodegenerative disorders such as Parkinson'sdisease (20). In 8 healthy volunteers and 7 patients with temporal lobeepilepsy, the internal anatomy of the hippocampal formation was welldefined. The alveus and fimbria were resolved from adjacent cellularlayers; the stratum radiatum and lacunosum were resolved from the cornuammonis. Atrophy, normal layer disruptions, and/or T1 and T2* deviationsclearly indicated the affected hippocampus in all seven patients studied(21). High resolution spectroscopic images (0.5 cc voxels) from 10healthy volunteers and 3 multiple sclerosis (MS) patients indicates thepotential for using the spatial variability of NAA, creatine, choline,and lactate across MS plaques for the diagnosis and understanding of thedisease (22). In high resolution NAA images of the whole brain, the lossof NAA from small stroke penumbra is dramatic (23). The high spatial andtemporal resolution detection and quantification of the amino acidsglutamate and glutamine are important for mechanistic studies anddiagnoses of metabolic disorders such as hepatic encephalopathy (24).High resolution spectroscopic imaging studies of 10 human subjects havebeen used to quantify gray and white matter metabolites in the wholebrain In-Vivo (25). The tuned TEM resonator has proven effective for theapplication of homogeneous, high resolution 3-D cerebral activationmapping (functional imaging) of the human brain at 175 MHz (26). Thepreceding results and those from studies in progress, demonstrate theeffectiveness of the tuned TEM resonator design for high field clinicalapplications.

Human in, ages and spectra obtained with an experimental 4.1 T MR systemindicate the advantages gained by performing clinical studies at higherB₀ fields. These advantages of S/N, spatial resolution, temporalresolution, chemical shift resolution and the magnetic susceptibilityenhancement of brain and other organ functions point to higher B₀ fieldsin clinical MR.

My new RF coil achieves these advantages, so it can replace coils of thepresent lumped element technologies which perform poorly for clinicalsized volumes at higher frequencies. The distributed technologypresented herein, making use of transmission lines and resonantcavities, perform well over the tested bandwidth of 70–175 MHz for humanhead and body coil applications. Beyond this, bench tested prototypesingle and double tuned large volume coils forecast successful use ofthe tuned TEM resonator at frequencies to 500 MHz, for human volumes,and even higher for smaller volumes.

REFERENCES

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It will be appreciated that the present invention can take many formsand embodiments. The true essence and spirit of this invention aredefined in the appended claims, and it is not intended that theembodiment of the invention presented herein should limit the scopethereof.

1. A magnetic resonance apparatus, comprising: an MR RF coil in the formof a tuned TEM resonator, said resonator having a cavity and a set oftransmission line segments; said coil having at least two wirelesslycoupled sections; and said at least two sections being separable,wherein only one section is powered.
 2. The apparatus of claim 1,wherein at least one of said two sections has an open window.
 3. Theapparatus of claim 2, wherein the section having an open window furthercomprises at least one opening allowing a subject to see and be accessedthrough said coil.
 4. The apparatus of claim 3, wherein said open windowhaving room for a subject having a large head.
 5. The apparatus of claim1, further comprising at least one cushioned head restraint.
 6. Theapparatus of claim 5, wherein said at least one head restraint providesa means of communication between a subject and an MR operator.
 7. Theapparatus of claim 6, wherein said at least one head restraint providesa means to communicate music to said subject.
 8. The apparatus of claim7, wherein said at least one head restraint provides sound suppression.9. The apparatus of claim 1, further comprising a subject input/outputdevice providing visual data from in front or behind of said coilrespectively; wherein said input/output device is selected from thegroup consisting of mirrors, prisms, video monitors, LCD devices, plasmadevices, and optical motion trackers.
 10. The apparatus of claim 9,wherein said input/output device is adjustable to a plurality ofpositions without moving the subject.
 11. The apparatus of claim 10,wherein said input/output device is mounted substantially within thecoil.
 12. The apparatus of claim 11, wherein said input/output device ismoveable about a track.
 13. The apparatus of claim 12, wherein theinput/output device is slideably adjustable to provide for a subject'sviewing.
 14. The apparatus of claim 13, wherein a channel positioned ina back plane of said coil provides access for a rear visual projectionsystem and general medical access.
 15. An RF coil for magneticresonance, comprising: said MR coil in the form of a tuned TEMresonator, said resonator having a cavity and a set of transmission linesegments, said coil comprising a plurality of sections; and saidplurality of sections being without hard electrical connections and ableto wirelessly couple electromagnetic energy between the plurality ofsections; and said plurality of sections being separable, wherein onlyone section is powered.
 16. The RF coil of claim 15, wherein said coilis a head coil.
 17. The RF coil of claim 16, wherein said coil is a limbcoil.
 18. The RF coil of claim 16, wherein said coil is a body coil. 19.The RF coil of claim 16, wherein is an animal research or veterinarycoils.
 20. The RF coil of claim 16, wherein said coil provides for aguiding means to assure mutual alignment between said plurality ofsections.
 21. A TEM coil for magnetic resonance; comprising: said MR REcoil providing a plurality of sections; and said plurality of sectionsbeing wirelessly connected; and said plurality of sections beingseparable, wherein only one section is powered.
 22. The TEM coil ofclaim 21, wherein said coil is a head coil.
 23. The TEM coil of claim21, wherein said coil is a limb coil.
 24. The TEM coil of claim 21,wherein said coil is a body coil.
 25. The TEM coil of claim 21, whereinsaid coil is for animal research or is a veterinary coil.
 26. The TEMcoil of claim 21, wherein said coil provides for a guiding means toassure mutual top and bottom section alignment.
 27. A magnetic resonanceapparatus, comprising: an MR RF coil in the form of a tuned TEMresonator, providing a plurality of sections; said plurality of sectionsbeing without electrical connections and wirelessly couplingelectromagnetic energy between the plurality of sections; and saidplurality of sections being separable, wherein only one section ispowered; and said coil providing a channel positioned in a back plane ofsaid coil to provide access for a rear visual projection system.
 28. Amethod of performing magnetic resonance imaging, comprising: providing acoil in the form of a tuned TEM resonator having a top section and abottom section; said top section and bottom section being separable andwirelessly connected, wherein only one section is powered; placing asubject within the bottom section; placing the top section inelectromagnetic coupling with the bottom section.
 29. The method ofclaim 28, wherein the top section has an open window.
 30. The method ofclaim 29, further comprising the steps of providing a plurality ofopenings in the top section to allow a subject to see through said coiland be accessed through said coil.
 31. The method of claim 30, whereinsaid open window allows room for a subject having a large head.
 32. Themethod of claim 28, further comprising the steps of providing at leastone cushioned device for head restraint.
 33. The method of claim 32,wherein said at least one cushioned device provides communicationsbetween a subject and an MR operator.
 34. The method of claim 33,wherein said at least one cushioned device provides music to saidsubject.
 35. The method of claim 34, wherein said at least one cushionprovides sound dampening.
 36. The method of claim 28, further comprisingthe step of inserting an optical system.
 37. The method of claim 36,wherein said optical system is adjustable to a plurality of positionswithout moving the subject.
 38. The method of claim 37, wherein saidoptical system is mounted generally within the coil.
 39. The method ofclaim 38, wherein said optical system is moveable about a tracktraversing from a first end of the coil to a second end of the coil. 40.The method of claim 39, wherein the optical system is slideablyadjustable to provide a subject's viewing.
 41. The method of claim 28,further comprising the steps positioning a channel in a back plane ofsaid coil to provide access for a rear visual projection system.
 42. Amagnetic resonance apparatus, comprising: an MR RF coil in the form of atuned TEM resonator, said resonator having a cavity and a set oftransmission line segments, said coil having at least two sections; saidat least two sections having a resonant electrical circuit; said atleast two sections being electromagnetically decoupled, wherein only onesection is powered; and said at least two sections being separable. 43.The apparatus of claim 42, further comprising a subject input/outputdevice providing visual data from in front and behind of said coilrespectively; wherein said input/output device is selected from thegroup consisting of mirrors, prisms, video monitors, LCD devices, plasmadevices, and optical motion trackers.